1. Field of the Invention
The present invention relates to a power supply source of a relatively low D.C. voltage to a circuit for controlling a switch or a load supplied by a relatively high voltage. The present invention more specifically relates to the supply of a high-voltage load control circuit upstream of a rectifying bridge, for example, providing a rectified voltage to a power converter of switched-mode power supply type.
2. Discussion of the Related Art
FIG. 1 very schematically shows in the form of blocks a first conventional example of provision of a relatively low supply voltage to a control circuit 1, in this example, via a switch K, of a load 2 (Q) supplied by a relatively high A.C. voltage Vac applied between terminals P and N. Terminals P and N correspond to A.C. input terminals of a fullwave rectifying bridge 3 having rectified output terminals E and M supplying a power converter 4 (CONV), for example, of switched-mode power supply type. In the example of FIG. 1, converter 4 provides several D.C. voltages of different levels (for example, two voltages Vs1 and Vs2) referenced to a ground G that may be different from output ground M of bridge 3.
The supply voltage of control circuit 1 is, for example, provided by a supply block 6 itself supplied from a voltage Vsaux provided by converter 4 by means of an auxiliary winding.
A problem is that the winding of provision of voltage Vsaux must be referenced to neutral N of the A.C. power supply, which must thus be isolated from ground G of the other output voltages of converter 4. This need for isolation within the actual converter 4, linked to the presence of an auxiliary winding referenced to the A.C. network, increases the converter complexity and bulk.
Further, the auxiliary winding alters the coupling and increases leakage inductances of the converter.
FIG. 2 shows a second conventional example of provision of a supply voltage to a control circuit 1 of a load 2 supplied by an A.C. voltage Vac. It shows fullwave rectifying bridge 3 and a converter of switched-mode power supply type 4′. The essential difference with respect to FIG. 1 is that the example of FIG. 2 uses a high-voltage capacitor CX upstream of rectifying bridge 3 rather than an auxiliary winding of converter 4′. Capacitor CX is connected by one of its electrodes to phase P of the A.C power supply and by the other one of its electrodes to a resistor R in series with a diode D and a capacitor C providing the supply voltage of circuit 1. A zener diode DZ is connected between the junction point of resistor R and diode D, and neutral N of the A.C. power supply. Zener diode DZ sets the value of the voltage across capacitor C, and thus of the low supply voltage of circuit 1. Diode D is used as a rectifying element, capacitor C being recharged one halfwave out of two of A.C. voltage Vac, since diode DZ conducts forward via capacitor CX.
The example of FIG. 2, which actually consists of forming a high non-dissipative impedance in series with capacitor C of provision of the supply of control circuit 1, poses the same problems of size and cost as the example of FIG. 1.